In many modern manufacturing processes, particularly in the semiconductor field, very narrow tolerances have to be observed for the surfaces to be processed. Parameters of interest in these processes are the planarity of the surface, its positive or negative slopes and the positions of maxima and minima. Apart from the static determination of the topography of a surface to be processed, process control requires the supervision of the modification with respect to time of that surface. Another desirable feature is a maximum measuring range since micrometer precision and better may be required. Also ranges of some 100 .mu.m may be covered.
For high precision measurements, interferometric processes are known where the phase difference (the path difference) between a measuring and a reference beam is evaluated. With these processes it is possible to produce a contour line of the entire measured surface, as in UK Pat. No. 1,538,811, or the measuring beam effects a point by point probing of the surface. However, with these known interferometric methods it is necessary that the phase relation between the two beams is not disturbed. In connection with highly scattering surfaces however, in particular those where the mean surface roughness has reached the order of the wavelength used, this condition is no longer satisfied. The field of application of these methods could be extended through infrared illumination, but this would include the problems observed in the optics, adjustment, and evaluation of interferometric methods in the infrared range. For the practical application of interferometric methods it is highly desirable to increase their measuring scope which is limited per se to be able to measure step heights of some 10 .mu.m on rough surface, by way of example.
An object of the present invention is to provide an interferometric measuring method and means of the above specified type permitting the precise measuring of the topography of rough surfaces.